The invention is related in general to the area of inspection systems. In particular, the invention provides a method and system for inspecting printed patterns on a flat substrate, such as liquid crystal displays (LCD), flat panel displays (FPD), organic light emitting diode (OLED) substrates, masks and semiconductor wafers during a corresponding manufacturing process.
Automatic optical inspection (AOI) is a critical step in a manufacturing process of LCD and semiconductor IC chips for diagnosing and improving the yield of the production, thus reducing the manufacturing cost. The fundamental performance of AOI is measured by two key specifications: speed and sensitivity of inspection. The advances of manufacturing technology have lead to higher speed of fabrication, a larger size of substrate, and smaller dimensions of printed patterns, all of which result in a more demanding need for AOI with higher speed and better sensitivity.
The amount of light required for acquiring images of sample surfaces is inversely proportional to the area of the imaging pixel size and the speed of inspection, and inevitably, the high speed and high resolution inspection for LCD or wafer need high efficient illumination optics. Increasing imaging lens numerical aperture (NA) proportionally to the imaging pixel size could compensate for the increased demand for more lighting, but has a negative impact on the requirements of mechanical precision of the system due to the reduction of depth of focus. In addition, the NA of the imaging lens is often limited by a large field of view that is necessary for inspecting large sample surfaces. Conventionally the fiber line lights, or LED line lights are used for illumination when a line scan CCD camera is used to scan the surface of samples. Such illumination method and apparatus work well for low-resolution inspections, for example, with an image pixel size of larger than 10 um. The illumination technique becomes inefficient for high-resolution inspection, for example, for imaging pixel sizes of less than 10 um. The disadvantages of using a fiber line light include the low efficiency (typically 50% from one end to another end) of the total effective transmission of the fiber bundle, largely due to the gap between individual fibers, reflections of end surface and transmission loss in fibers. Fiber bundles are normally used with a lamp based light source. A lamp source has a short life time on the order of 1000 hours. Although the lamps can be replaced easily and cost effectively, the down time associated with replacing the lamp often includes re-calibration of the inspection tool, lasting several hours. In return, this has a significant impact on the cost of production where the production line is running continuously at high speed. Another disadvantage of using a lamp-based light source is that lamps generate light at a wide range of wavelengths. Since only a small range of the wavelengths is used for imaging, the light efficiency is further reduced when filters are used.
LEDs are more attractive as light sources for AOI. Their advantages include the high efficiency in generating light at desired wavelengths and very long lifetime in the order of tens of thousands of hours. However, the total amount of light output by a single LED chip is still much lower than a lamp, for example, a current high power single LED light source is rated at around a few watts, while most lamp light sources are rated at a few hundreds of watts. A conventional method of increasing the light intensity of LEDs for AOI is to densely package a large number of LED chips on a single printed circuit board, for example, the LED line lights supplied by Stockeryale (model of COBRA LED line light). However, the densely packaged LED chips result in excessive heat generation and a large package size due to the requirements of a large heat sink. Furthermore, the brightness (defined as the light power per unit area per unit solid angle) of the light source does not increase when more LED chips are added. The majority of light output is wasted when the optical invariant (defined by the product of the light emitting area and the solid angle of light output) of the light source largely exceeds the optical invariant of the imaging optics (defined by product of the field of view and NA).
Another disadvantage of using a fiber line array or LED chip array is the poor uniformity in illumination over a large field of view. The uniformity of illumination is largely determined by the transmission uniformity of each individual fibers, or variation between LED chips, and the geometric alignment errors of each chip.
One of the challenges associated with inspecting large samples, such as a 7th generation LCD glass substrate well over 2000 mm×2000 mm in size, is the footprint of the inspection tools. Fabrication clean rooms are expensive to construct and operate, thus smaller footprint equipments are highly desired.
Therefore, there is a need for highly efficient, long lifetime, compact and cost effective illumination method and apparatus for high resolution AOI of LCD glass substrate and semiconductor wafers.